Creep-resistant nickel-chromiumcobalt alloy



United States Patent 3,107,999 CREEP-RESISTANT NICKEL-CERGMIUM- CGBALT ALLGY John Henry Gittns, Clifton, near Preston, England, as-

signor to The International'Nickel Company, inc,

New York, N.Y., a corporation of Delaware N0 Drawing. Filed'Nov. 1, 1960, Ser. No. 6,440 Claims priority, application Great Britain Nov. 4, 1959 1 Claim. (Cl. 75-171) The present invention relates to alloys resistant to the deleterious effects of high stress and high temperatures and, more particularly, to heat-resistant, nickelchromium-cobali alloys.

It is well known that gas turbine blades and other articles and parts that are required to be resistant to creep when subjected to stress at elevated temperatures are commonly made of alloys in which the principal constituent is nickel or nickel plus cobalt and which also contain chromium, aluminum and titanium, these last two elements forming a precipitable phase with some of the nickel. Other elements, for example, molybdenum and tungsten, can also be present with beneficial effects on the creep resistance and it has also been found that melting the alloys under vacuum may further improve the properties.

Heretofore, the art has been unable to properly correlate each of the foregoing factors in vacuum-treated, hot-workable alloys in order to achieve the advantageous optimum combination of desirable alloy characteristics. Although attempts were made to provide alloys having an optimum combination of characteristics, none, as far as I am aware, was entirely successful when carried into practice commercially on an industrial scale.

It has now been discovered that by means of a specifically cor-related composition, alloys can now be provided which exhibit an optimum combination of advantageous alloying characteristics.

It is an object of the present invention to provide a novel, vacuum-treated, hot-workable alloy.

Another object of the invention is to provide a novel process for producing a novel, hot-workable alloy.

Other objects and advantages will become apparent from the following description.

Generally speaking, the present invention contemplates hot-workable alloys having particularly good creep resistance which contain about 3% to about 7% cobalt, about 13% to about 17% chromium, about 3% to about 7% molybdenum, about 0.10% to about 0.2% carbon and also titanium and aluminum in such amounts that the Ti/Al ratio is from 0.1 to 3.0 and the total content of 'titanium and aluminum (i.e., Ti plus Al) is given by the formula:

the balance, apart from impurities, being nickel. Advantageously, the alloys also contain small effective amounts of boron and/ or zirconium in amounts up to about 0.05% boron and 0.2% zirconium in order to enhance the creep resistance and/ or ductility of the alloys. The alloys can also contain from 0% to about 0.5% silicon, from 0% to 0.5% manganese and from 0% to 1.0% iron.

The invention is based on the discovery that in alloys containing cobalt, chromium and molybdenum which have been vacuum melted or treated by holding in the molten state under vacuum, the creep resistance depends not only on the basic composition of the alloys but also both on the ratio of the content of titanium (Ti) to that of aluminum (Al) and on the total content of titanium 3,107,990 Patented Oct. 22., 1953 series of alloys having a given Ti/Al ratio, the life-to rupture under given conditions of stress and temperature passes through a maximum as the Ti plus Al content is increased; and the Ti plus Al content at which this maximum occurs increases as the Ti/Al ratio is increased. Accordingly, for the highest creep resistance, the Ti/Al ratio must be correlated with the Ti plus Al content.

Correlation of the Ti/Al ratio with the Ti plus Al content according to the formula ensures that the life-torupture lies at or near the maximum value obtainable for the particular ratio or content used. As the Ti/Al ratio increases, the maximum life-to-rupture also increases and advantageously, therefore, the Ti/Al ratio is at least 1.5. The upper limit of 3.0 is imposed by the increasing difficulty of working the alloys as the total amount of titanium and muminum present also increases.

The vacuum melting or vacuum treatment of th alloys is carried out at a pressure not exceeding 100 micron of mercury, advantageously, at five microns of mercury or less. To obtain the best properties the molten alloy is subjected to such a pressure for at least five minutes and, advantageously, for at least ten minutes at a temperature of 1400 C.-1600C. More advantageously, the alloys are first vacuum melted and then held under vacuum before casting, but the invention also includes alloys which have been melted in air and then held under vacuum in the molten state. Once the alloys have been vacuum melted or maintained in the molten state under vacuum or both, it makes little difference to their properties whether they are cast in vacuum or in air.

Ingots cast from the vacuum-treated metal can be hot worked to produce bar, and to develop their creep-resistin-g properties the alloys are subjected to a precipitationhardening treatment consisting of solution heating at a temperature in the range 1l50 C. to 1250 C., followed by ageing at a temperature in the range 700 C. to 1100 C. The solution temperature should be as high as possible, the limit being set by the desirability of avoiding excessive grain-growth. The ageing temperature should be higher than the anticipated service temperature of components made from the bar.

For the purpose of giving those skilled in the art'a better understanding of the invention by way of example, a number of alloys of the same base composition, apart from their titanium and aluminum contents, were made by melting in vacuum under a pressure of one micron of mercury, held in the molten state at 1500 C. (i.e., about 2730 F.) for 15 minutes under the same pressure and then vacuum cast under the same pressure to ingots 2% inches in diameter. These ingots were extruded to inch diameter bar which was heat treated by solution heating for 1 /2 hours at 1200 C., air-cooling to room temperature, aging for 16 hours at 1050 C. and again air-cooling. Standard creep test pieces having a gage length of 1% inches were machined from the heat treatedbars and tested under a stress of 7 tons per square inch (i.e., 15,680 pounds Percent;

o Cr Si 3 Table Ti-l-Al, 7+% 6+% Lite to Alloy No Ti/Al percent (TilAl) (Ti/A1) rupture (hours) It is to be noted that each of the alloys in accordance with the invention exhibited lives-to-rupture under the stated conditions of more than 100 hours and even more than 150 hours. On the other hand, alloys not in accordance with the present invention had lives-to-rupture less than 100 hours under the stated conditions.

The present invention is particularly applicable to alloys and the production of alloys having an optimum combination of hot workability, heat resistance and resistance to stress for use as components in gas turbine structures such as turbine blades, stator blades, guide vanes, etc. In addition, the alloys of the present invention can be employed as bolts, fastenings, etc., in structures subject to the deleterious action of high temperature and high stress.

Although the present invention. has been described in conjunction with preferred embodiments, it is to be under- ZZ I g stood that modifications and variations may be resorted to Without departing from the spirit and scope of the invention, as those skilled in the art will readily understand.

Such modifications and variations are considered to be Within the purview and scope of the invention and appended claim.

I claim:

A vacuum treated alloy for structural members subjected in use to a combination of high stress and high temperature consisting essentially of about 5% cobalt, about 1 15% chromium, about 5% molybdenum, about 0.15% carbon, about 0.05% zirconium, about 0.01% boron, about 0.3%silic0n and aluminum and titanium in a ratio of titanium to aluminum of 0.1 to 3.0 and a total per:

centage of titanium plus aluminum in excess of 6 plus.

References Cited in the file of this patent FOREIGN PATENTS 548,778 Canada Nov. 12, 1957 

